1. Field of the Invention
The present invention relates to an apparatus and a method for processing reduced bandwidth Envelope Tracking (ET) and Digital Pre-Distortion (DPD).
2. Description of the Related Art
As a mobile communication standard passes through a 2nd generation (2G) and a 3rd generation (3G) and evolves to a 4th generation (4G), a wireless communication system processes a signal of a high data rate and requires a transmission signal in a broadband and a complicated modulation method. For example, a transceiver processes a broadband signal and a signal having a high Peak to Average Power Ratio (PAPR). Accordingly, a transceiver of a wireless communication system has a high efficiency, a broadband signal processing capability, and a linear amplify characteristic.
To achieve these characteristics, a wireless communication system uses a Power Amplifier (PA) of a polar modulation scheme.
In a case of using the polar modulation scheme, a transmitter separates a phase component and an envelope component of an input signal. Thereafter, the transmitter up-converts the phase component to a Radio Frequency (RF) signal to provide the input signal of the PA, and modulates the envelope component using a Supply Modulator (SM) to provide the envelope component to a collector/drain source of the PA. The PA amplifies the up-converted phase component provided via an input terminal using the modulated envelope component provided via the collector/drain terminal to maintain a high efficiency and linearity of a high PAPR signal.
In a 3G/4G communication system, due to an increase in an amount of used by a multimedia service, a power consumption issue has emerged as an important problem.
Since a power amplification efficiency is closely related to heat generated from a power device, when the power amplification efficiency is poor, much heat is generated which not only deteriorates a characteristic and durability of the device but also increases cooling system costs. Accordingly, implementing a linear PA having a high power amplification efficiency lowers system and maintenance costs, and guarantees the performance and durability of the power device.
A PA of the related art has at least two shortcomings A first shortcoming is illustrated with respect to FIG. 1.
FIG. 1 is a graph illustrating a PA in a case where a supply voltage is a fixed value according to the related art.
Referring to FIG. 1, in the PA, since a supply voltage Vdd is a fixed value, residual power after generating a signal is transformed to heat and thus is wasted, and an additional cooling solution for resolving high heat generation is required, which increases hardware costs. To address this issue, various power amplification technologies, such as Envelope Tracking (ET), Envelope Elimination and Restoration (EER), Doherty, and the like, have been suggested.
FIG. 2 is a graph illustrating a PA by an ET supply voltage according to the related art. FIG. 3 illustrates an ET PA according to the related art.
Referring to FIGS. 2 and 3, unlike the PA of the related art, the ET of the related art uses an SM and not a fixed voltage. The SM adaptively applies a supply voltage depending on an envelope of an RF input signal, thereby raising power efficiency and minimizing power consumption. Since ET has a small hardware complexity compared to a Doherty PA, ET draws attention when an operator desires to obtain a high efficiency with small complexity as in a terminal.
A second problem of the PA of the related art is that Out Of Band (OOB) spectrum radiation is high and an Adjacent Channel Leakage Ratio (ACLR) is lowered, which acts as an interference to other users. To address this problem, a Digital Pre-Distortion (DPD) technology is suggested.
DPD allows an overall input/output curve to form linearity by distorting a relevant signal in advance depending on non-linearity of a PA before amplifying the signal.
FIG. 4 is a graph illustrating DPD for PA linearization according to the related art.
Referring to FIG. 4, an output power curve for input power of a PA gradually saturates at high power as in a first line 401. Accordingly, when the PA pre-distorts an input signal in a baseband as in a second line 403 according to DPD, the linearity of a final PA output improves as in a third line 405.
FIG. 5 is a block diagram illustrating a transmitter structure that combines an open loop DPD with an ET that applies a 1-dimensional (1D) Shaping Function (SF) according to the related art.
Referring to FIG. 5, a first path in a transmitter via a Crest Factor Reduction (CFR) 502, a DPD 504, an In-phase/Quadrature-phase (I/Q) mismatch unit 506, an Inverse Sinc Filter 508, a first Digital Analog Converter (DAC) 510, a first Low Pass Filter (LPF) 512, an up converter 514, a Driving Amplifier (DA) 516, and a PA 518 represents a data path of an I/Q signal. A second path via a fractional delay filter 520, an envelope detector 522, an envelope converter 524, a second DAC 526, a second LPF 528, and a Supply Modulator (SM) 530 represents an envelope path of an I/Q signal.
As described above, in a case of including the data path and the envelope path, the transmitter requires time-axis alignment on a sub-sample basis as well as delay on a sample basis in order to adjust a delay difference between the data path and the envelope path. For this purpose, the transmitter uses the fractional delay filter 520.
Generally, it is known that a bandwidth of an envelope signal is three to four times greater than a bandwidth of an I/Q signal. Accordingly, for the SM 530, to meet a wide bandwidth of an envelope signal, a hybrid type simultaneously including a linear amplifier and a switching amplifier is used, which increases hardware complexity. In a case where an uplink and a downlink are not clearly separated in a wireless communication system that uses a Frequency Domain Duplex (FDD), an uplink signal leaks to a downlink by a wide bandwidth of an envelope signal to act as a receive (Rx) band noise and influence sensitivity of a receiver.
FIG. 6 is a graph illustrating use of a 1D SF for reducing bandwidth of an envelope signal according to the related art.
Referring to FIG. 6, assuming that a minimum voltage for a normal operation allowing the PA 518 to obtain an appropriate gain and efficiency is a knee voltage, the knee voltage corresponds to a first horizontal line 601 (value=0.37). In a case of using this input/output transfer function, a drastic change of an envelope signal occurs in the vicinity of the knee voltage, and as a result, a high frequency component occurs, which deteriorates a spectrum performance and consequently, deteriorates an Adjacent Channel Power (ACP) performance. Therefore, when a curve having a smooth transition, such as a second line 603, is used, power efficiency and an ACP performance are balanced, and ACP performance deterioration may be mitigated to some degree. However, according to an analysis of an experiment, in a case of maintaining a knee voltage at a level of 0.1-0.3 in order to maintain power efficiency, a spectrum performance is not satisfactory and a desired bandwidth reduction effect is difficult to obtain. In contrast, when the knee voltage is raised by about 0.4-0.5 in order to improve the spectrum performance, the spectrum performance improves but power efficiency falls down excessively, and as a result, utility of using ET falls down.
FIG. 7 is a graph illustrating a time domain envelope shape having a reduced bandwidth according to the related art.
Referring to FIG. 7, when using an LPF, which is a technique for reducing a bandwidth of an envelope signal, a filtered envelope (i.e., a dotted line 703) may not properly follow the original signal envelope 701. In a case where a filtered signal LPF {e(t)} 703 is smaller than an original signal e(t) 701, a transmitter cannot provide sufficient Vdd to a PA, and as a result, a Radio Frequency Integrated Circuit (RFIC) generates oscillation and a frequency component may be distorted at an undesired position.
FIG. 8 is a block diagram illustrating a bandwidth reduction according to the related art.
Referring to FIG. 8, to address the aforementioned issue of frequency distortion, a technique where a transmitter repeatedly applies a half-wave rectifier 810 and an LPF2 820 to reduce a bandwidth of an envelope signal is suggested. However, such a technique may represent a shape similar to repeated clipping and filtering, which is a kind of a PAPR reducing method. For example, the technique of reducing a bandwidth of an envelope signal by repeatedly applying the half-wave rectifier 810 and the LPF2 820 has an initial pass and a repetitive pass, and an LPF1 815 and an LPF2 820 are LPFs of more than 100 taps and correspond to a substantially unusable algorithm with consideration of hardware complexity and a delay.
FIGS. 9A and 9B are graphs illustrating a requirement of a predefined repetitive path in reducing bandwidth of an envelope signal according to the related art.
Referring to FIGS. 9A and 9B, 4-5 times of a repetitive pass process should be performed to make a bandwidth of an envelope signal smaller than a bandwidth of an original signal. However, the bandwidth of the envelope signal is not completely removed, and an excessive hardware complexity is imperative for a plurality of repetitions.
Therefore, a need exists for a method and an apparatus capable of efficiently reducing a bandwidth of an envelope in aspects of hardware complexity and delay when using ET.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.